There is too many of these, so we should better skim through them fast.
Actually, we already talked about them. It's the last argument to the bind command. So, they are those listed in the keybinding manager. So if you enter move-cursor-left command, it will move your cursor left - by a single character, making this a little awkward, but it's useful if you sometimes want to easily invoke an action and you don't want to waste a key for it.
Actually, actions could theoretically take arguments too. This is currently implemented only for the goto-url action, which can take the location it should go at as a parameter (otherwise it opens the standard well-known dialog as if you pressed g in the default keymap).
Regarding the mysterious "(almost)" hinted above, you can never invoke the "quit" action from the exmode - if you type it there, "really-quit" is invoked instead.